Oxytocin (OT) is a cyclic nona-peptide that mediates its physiological actions through activation of the oxytocin receptor (OT-R), a cell membrane receptor belonging to the class of G protein-coupled receptors that is similar to arginine vasopressin receptors. Among other roles in the body, OT causes the contraction of the uterus of mammals during labor. Repeated, concerted and regular contraction of the uterus will cause the dilation of the cervix, the rupture of fetal membranes and lead to expulsion of the fetus. Premature labor is when these contractions occur before the normal term of pregnancy. Preterm increase of uterine activity is the most common expression of preterm labor.
Premature labor leads to undesired premature birth, a serious health problem that remains the major cause of perinatal mortality and severe morbidity, especially respiratory distress syndrome, intraventricular haemorrhage, bronchopulmonary dysplasia and necrotising enterocolitis that are tar more common in preterm than in term infants. Long-term impairments such as cerebral palsy, visual impairment and hearing loss are also more common in preterm infants. Nowadays, preterm birth remains the leading cause of infant mortality and morbidity in industrialized nations. Neonatal intensive care of premature babies is very costly. The actual costs are even higher to society when taking into consideration the healthcare provision of preterm child birth-related ailments, such as respiratory distress syndrome, heart conditions, cerebral palsy, epilepsy, and severe learning disabilities. Thus, managing preterm labor is a significant concern in the field of obstetrics.
The OT/OT-R system plays a vital role in initiating labor in mammals, in particular in humans. The density of OT-R increases markedly in the myometrium before the onset of and during labor. Also, it is thought that the local OT peptide hormone concentration increases significantly before parturition in humans. The high circulating concentrations of progesterone induce uterine quiescence while the uterus acquires contractile ability. Shortly before term, plasma progesterone concentrations fall, OT-R expression in the uterus increases, OT is released and uterine contractile activity increases. At term, the contractions rise to a crescendo, resulting in delivery as a result of two interacting positive feedback loops. The first is a local uterine loop; within the uterus itself contractile prostaglandins are produced and released in response to OT and uterine contractions. These prostaglandins may play a further role in cervical ripening and weakening of fetal membranes. The second loop involves the hypothalamus; in response to uterine contractions and vaginal and cervical distension, monocellular oxytocin neurons in the hypothalamus increase their activity resulting in the release of OT from their axon terminals in the posterior pituitary. The released OT acts upon the uterus both to stimulate the further production of prostaglandins and to contribute further to the contractions of the uterus.
Another potential benefit of antagonizing OT-R is in the field of Assisted Reproductive Technology (ART), Although many efforts have been made to improve the results of assisted reproduction over the last decades, the overall effectiveness of in vitro fertilization (IVF) technique still remains limited. A variety of factors can influence success rates after IVF. Transfer of the embryo is an important factor influencing the outcome of the fertility treatment. ART consists first in performing a controlled ovarian hyperstimulation (COH) for stimulating the growth of several follicles, allowing retrieval of several oocytes for IVF. COH is associated with supra-physiological estradiol levels and it has been shown that uterine contractile activity is increased in IVF patients at the time of embryo transfer as compared with a spontaneous menstrual cycle.
Uterine contractions constitute one of the most fundamental components of uterine receptivity, because contractile activity of the uterus plays an important role in embryo implantation. Excessive uterine contractions may decrease the embryo implantation rate in an IVF cycle because contractile activity might expel embryos from the uterus. To date, treatment strategies used to reduce uterine contractions before embryo transfer, such as the use of beta agonists or non-steroid anti-inflammatory drugs, have not provided sufficient benefit
In addition, systemic and endometrial levels of OT, as well as OT-R expression, are strongly influenced by estradiol, e.g., in non-pregnant women, the highest level of expression of OT-R is observed at mid-menstrual cycle and in pregnant women near term.
For at least these reasons, it is believed that reducing uterus contractions at the time of embryo transfer by the administration of an OT-R and/or V1a antagonist may increase embryo implantation rate and thus pregnancy rate in ART.
Thus, blocking the effect of OT by antagonizing OT-R might represents an attractive modality tor the treatment of diseases related to the OT-R activity, in particular preterm labor and embryo implantation failure due to uterine contractions.
Tocolytics, i.e. uterus relaxing agents, have been used in clinical studies for the pharmaceutical treatment of preterm labor. Most of these agents are used oil-label. They have shown very limited efficacy, if any, in prolonging gestation and have not shown any clear demonstration of improvement of neonate outcome. In addition, many tocolytics are often associated with unwanted adverse effects on women, fetus or neonate. Such tocolytics include beta-2-adrenergic agonists, prostaglandin synthesis inhibitors, magnesium sulfate, nitric acid donors and calcium channel blockers. Beta-2-adrenergic agonists such as ritodrine or terbutaline cause a number of cardiovascular and metabolic side effects including maternal tachycardia, palpitations, hypotension, altered thyroid function and fetal and neonatal hypoglycemia, tachycardia.
The calcium channel blocker nifedipine is also used to try to stop contractions. Some of the possible side effects from this medicine include facial flushing, headache, nausea, palpitations, and lightheadedness. The total prostaglandin synthesis inhibitor (NSAID) indomethacin has also been used, but it can also have serious effects on the fetus, e.g., constriction of ductus arteriosus, pulmonary hypertension, decrease in renal function with oligohydramnios, intraventricular hemorrhage, hyperbilirubinemia, necrotizing enterocolitis, and also side effects tor the mother, e.g., abdominal discomfort, nausea, vomiting, depression and dizzy spells. Sulindac, another NSAID, has a side effect profile similar to indomethacin. Meta-analyses conducted on magnesium sulfate have failed to support it as a tocolytic. Women reported side effects such as flushing, lethargy, headache, muscle weakness, pulmonary edema and cardiac arrest. Further, a newborn who has been exposed to magnesium sulfate may exhibit lethargy, hypotonia, respiratory depression, bone problems, osteopenia and fractures. The FDA is now advising healthcare professionals against using magnesium sulfate injection for longer than 5-7 days to stop preterm labor in women.
Another pharmaceutical, atosiban, a dual vasopressin V1a receptor and OT-R antagonist, is marketed in the EU and is used to stop contractions and delay preterm delivery by a few days. Atosiban is a peptide that is not orally bioavailable and must be administered parenterally. It degrades rapidly in circulation by enzymes and its use is limited to a maximum of 48 hours.
Orally active small molecule antagonists that are selective for the OT-R have been developed in an attempt to overcome these problems. Specifically. Non-peptide OT-R antagonists were developed such as pyrrolidine derivatives (WO 01/72705, WO 02/102799, WO 2002/07474, WO 2004/005249).
Pyrrolidine derivatives, as mixtures of isomers, arc disclosed for use as oxytocin antagonists in WO 2004/005249. No suitable conditions of crystallization of pure (3Z,5S)-5-(hydroxymethyl)-1-[(2′-methyl-1,1-biphenyl-4-yl)carbonyl]pyrrolidin-3-one O-methyloxime or of the crude isomeric mixture were reported therein.
There remain significant unmet needs for efficient and orally selective OT-R antagonist for the treatment of diseases related to the OT-R activity, in particular preterm labor, in particular, there is a need for an orally administrable pharmaceutically effective product, which is rapidly absorbed, has a half-life long enough to support once daily administration, and is safe for mother and fetus in a prolonged maintenance treatment over several weeks until the pregnancy comes to term.